Robust erasure detection and erasure-rate-based closed loop power control

ABSTRACT

Techniques for performing erasure detection and power control for a transmission without error detection coding are described. For erasure detection, a transmitter transmits codewords via a wireless channel. A receiver computes a metric for each received codeword, compares the computed metric against an erasure threshold, and declares the received codeword to be “erased” or “non-erased”. The receiver dynamically adjusts the erasure threshold based on received known codewords to achieve a target level of performance. For power control, an inner loop adjusts the transmit power to maintain a received signal quality (SNR) at a target SNR. An outer loop adjusts the target SNR based on the status of received codewords (erased or non-erased) to achieve a target erasure rate. A third loop adjusts the erasure threshold based on the status of received known codewords (“good”, “bad”, or erased) to achieve a target conditional error rate.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a continuation of patentapplication Ser. No. 10/890,717 entitled “ROBUST ERASURE DETECTION ANDERASURE-RATE-BASED CLOSED LOOP POWER CONTROL” filed Jul. 13, 2004,pending, and assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND

I. Field

The present invention relates generally to data communication, and morespecifically to techniques for performing erasure detection and powercontrol in a wireless communication system.

II. Background

A wireless multiple-access communication system can simultaneouslysupport communication for multiple wireless terminals. Each terminalcommunicates with one or more base stations via transmissions on theforward and reverse links. The forward link (or downlink) refers to thecommunication link from the base stations to the terminals, and thereverse link (or uplink) refers to the communication link from theterminals to the base stations.

Multiple terminals may simultaneously transmit on the reverse link bymultiplexing their transmissions to be orthogonal to one another. Themultiplexing attempts to achieve orthogonality among the multiplereverse link transmissions in time, frequency, and/or code domain.Complete orthogonality, if achieved, results in the transmission fromeach terminal not interfering with the transmissions from otherterminals at a receiving base station. However, complete orthogonalityamong the transmissions from different terminals is often not realizeddue to channel conditions, receiver imperfections, and so on. The lossin orthogonality results in each terminal causing some amounts ofinterference to other terminals. The performance of each terminal isthen degraded by the interference from all other terminals.

On the reverse link, a power control mechanism may be used to controlthe transmit power of each terminal in order to ensure good performancefor all terminals. This power control mechanism is normally implementedwith two power control loops, which are often called an “inner” loop andan “outer” loop. The inner loop adjusts the transmit power of a terminalsuch that its received signal quality (SNR), as measured at a receivingbase station, is maintained at a target SNR. The outer loop adjusts thetarget SNR to maintain a desired block error rate (BLER) or packet errorrate (PER).

The conventional power control mechanism adjusts the transmit power ofeach terminal such that the desired block/packet error rate is achievedfor the reverse link transmission from the terminal. An error detectioncode, such as a cyclic redundancy check (CRC) code, is typically used todetermine whether each received data block/packet is decoded correctlyor in error. The target SNR is then adjusted accordingly based on theresult of the error detection decoding. However, an error detection codemay not be used for some transmissions, e.g., if the overhead for theerror detection code is deemed excessive. A conventional power controlmechanism that relies on an error detection code cannot be used directlyfor these transmissions.

There is therefore a need in the art for techniques to properly adjusttransmit power for a transmission when error detection coding is notused.

SUMMARY

Techniques for performing erasure detection and power control for atransmission on a “physical” channel (e.g., a control channel or a datachannel) that does not employ error detection coding are describedherein. Data is transmitted as “codewords” on the physical channel,where each codeword may be a block of coded or uncoded data.

For erasure detection, a transmitting entity (e.g., a wireless terminal)transmits codewords on the physical channel and via a wireless channelto a receiving entity (e.g., a base station). The base station computesa metric for each received codeword, as described below, and comparesthe computed metric against an erasure threshold. The base stationdeclares each received codeword to be an “erased” codeword or a“non-erased” codeword based on the comparison result. The base stationdynamically adjusts the erasure threshold to achieve a target level ofperformance, which may be quantified by a target conditional error ratethat indicates the probability of a received codeword being decoded inerror when declared to be a non-erased codeword. The erasure thresholdmay be adjusted based on received known codewords, which are receivedcodewords for known codewords transmitted by terminals in communicationwith the base station, as described below. The adjustable erasurethreshold can provide robust erasure detection performance in variouschannel conditions.

A power control mechanism with three loops (an inner loop, an outerloop, and a third loop) may be used to control the transmit power forthe physical channel. The inner loop adjusts the transmit power for thephysical channel to maintain the received SNR at or near a target SNR.The outer loop adjusts the target SNR based on the status of receivedcodewords (erased or non-erased) to achieve a target erasure rate, whichis the probability of declaring a received codeword as an erasedcodeword. The third loop adjusts the erasure threshold based on thestatus of received known codewords (“good”, “bad”, or erased) to achievethe target conditional error rate. The target erasure rate and thetarget conditional error rate are two measures of performance for thephysical channel.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 shows a wireless multiple-access communication system;

FIG. 2 shows a power control mechanism with three loops;

FIGS. 3A and 3B show a process for updating the second and third loopsfor the power control mechanism shown in FIG. 2;

FIG. 4 shows data and control channels for a data transmission scheme;and

FIG. 5 shows a block diagram of a base station and a terminal.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a wireless multiple-access communication system 100. System100 includes a number of base stations 110 that support communicationfor a number of wireless terminals 120. A base station is a fixedstation used for communicating with the terminals and may also bereferred to as an access point, a Node B, or some other terminology.Terminals 120 are typically dispersed throughout the system, and eachterminal may be fixed or mobile. A terminal may also be referred to as amobile station, a user equipment (UE), a wireless communication device,or some other terminology. Each terminal may communicate with one ormore base stations on the forward and reverse links at any given moment.This depends on whether the terminal is active, whether soft handoff issupported, and whether the terminal is in soft handoff. For simplicity,FIG. 1 only shows transmissions on the reverse link. A system controller130 couples to base stations 110, provides coordination and control forthese base stations, and further controls the routing of data for theterminals served by these base stations.

The erasure detection and power control techniques described herein maybe used for various wireless communication systems. For example, thesetechniques may be used for a Code Division Multiple Access (CDMA)system, a Time Division Multiple Access (TDMA) system, a FrequencyDivision Multiple Access (FDMA) system, an orthogonal frequency divisionmultiple access (OFDMA) system, and so on. A CDMA system uses codedivision multiplexing, and transmissions for different terminals areorthogonalized by using different orthogonal (e.g., Walsh) codes for theforward link. The terminals use different pseudo-random number (PN)sequences for the reverse link in CDMA and are not completely orthogonalto one another. A TDMA system uses time division multiplexing, andtransmissions for different terminals are orthogonalized by transmittingin different time intervals. An FDMA system uses frequency divisionmultiplexing, and the transmissions for different terminals areorthogonalized by transmitting in different frequency subbands. An OFDMAsystem utilizes orthogonal frequency division multiplexing (OFDM), whicheffectively partitions the overall system bandwidth into a number oforthogonal frequency subbands. These subbands are also commonly referredto as tones, sub-carriers, bins, and frequency channels. An OFDMA systemmay use various orthogonal multiplexing schemes and may employ anycombination of time, frequency, and/or code division multiplexing.

The techniques described herein may be used for various types of“physical” channels that do not employ error detection coding. Thephysical channels may also be referred to as code channels, transportchannels, or some other terminology. The physical channels typicallyinclude “data” channels used to send traffic/packet data and “control”channels used to send overhead/control data. A system may employdifferent control channels to send different types of controlinformation. For example, a system may use (1) a CQI channel to sendchannel quality indicators (CQI) indicative of the quality of a wirelesschannel, (2) an ACK channel to send acknowledgments (ACK) for a hybridautomatic retransmission (H-ARQ) scheme, (3) a REQ channel to sendrequests for data transmission, and so on. The physical channels may ormay not employ other types of coding, even though error detection codingis not used. For example, a physical channel may not employ any coding,and data is sent “in the clear” on the physical channel. A physicalchannel may also employ block coding so that each block of data is codedto obtain a corresponding block of coded data, which is then sent on thephysical channel. The techniques described herein may be used for anyand all of these different physical (data and control) channels.

For clarity, the erasure detection and power control techniques arespecifically described below for an exemplary control channel used forthe reverse link. Transmissions from different terminals on this controlchannel may be orthogonally multiplexed in frequency, time, and/or codespace. With complete orthogonality, no interference is observed by eachterminal on the control channel. However, in the presence of frequencyselective fading (or variation in frequency response across the systembandwidth) and Doppler (due to movement), the transmissions fromdifferent terminals may not be orthogonal to one another at a receivingbase station.

Data is sent in blocks on the exemplary control channel, with each blockcontaining a predetermined number of (L) data bits. Each data block iscoded with a block code to obtain a corresponding codeword or coded datablock. Since each data block contains L bits, there are 2^(L) possibledifferent data blocks that are mapped to 2^(L) possible codewords in acodebook, one codeword for each different data block. The terminalstransmit codewords for the data blocks on the control channel.

A base station receives the codewords transmitted on the control channelby different terminals. The base station performs the complementaryblock decoding on each received codeword to obtain a decoded data block,which is a data block deemed most likely to have been transmitted forthe received codeword. The block decoding may be performed in variousmanners. For example, the base station may compute a Euclidean distancebetween the received codeword and each of the 2^(L) possible validcodewords in the codebook. In general, the Euclidean distance betweenthe received codeword and a given valid codeword is shorter the closerthe received codeword is to the valid codeword, and is longer thefarther away the received codeword is from the valid codeword. The datablock corresponding to the valid codeword with the shortest Euclideandistance to the received codeword is provided as the decoded data blockfor the received codeword.

As an example, the L data bits for a data block may be mapped to acodeword containing K modulation symbols for a particular modulationscheme (e.g., BPSK, QPSK, M-PSK, M-QAM, and so on). Each valid codewordis associated with a different set of K modulation symbols, and the2^(L) sets of modulation symbols for the 2^(L) possible valid codewordsmay be selected to be as far apart (in Euclidean distance) from eachother as possible. A received codeword would then contain K receivedsymbols, where each received symbol is a noisy version of a transmittedmodulation symbol. The Euclidean distance between the received codewordand a given valid codeword may be computed as: $\begin{matrix}{{{d_{i}(k)} = {\frac{1}{K}{\sum\limits_{j = 1}^{K}( {{{\hat{s}}_{k}(j)} - {s_{i}(j)}} )^{2}}}},} & {{Eq}\quad(1)}\end{matrix}$

-   -   where Ŝ_(k)(j) is the j-th received symbol for received codeword        k;    -   S_(i)(j) is thej-th modulation symbol for valid codeword i; and    -   d_(i)(k) is the Euclidean distance between received codeword k        and valid codeword i.

Equation (1) computes the Euclidean distance as the mean-squared errorbetween the K received symbols for the received codeword and the Kmodulation symbols for the valid codeword. The data block correspondingto the valid codeword with the smallest d_(i)(k) is provided as thedecoded data block for the received codeword.

Without an error detection code, there is no direct way to determinewhether the block decoding of a given received codeword is correct or inerror, and that the decoded data block is indeed the transmitted datablock. A metric may be defined and used to provide an indication of theconfidence in the decoding result. In an embodiment, a metric may bedefined as follows: $\begin{matrix}{{{m(k)} = \frac{d_{n\quad 1}(k)}{d_{n\quad 2}(k)}},} & {{Eq}\quad(2)}\end{matrix}$

-   -   where d_(n1)(k) is the Euclidean distance between received        codeword k and the nearest valid codeword;    -   d_(n2)(k) is the Euclidean distance between received codeword k        and the next nearest valid codeword; and    -   m(k) is the metric for received codeword k.

If the received codeword is much closer to the nearest codeword than thenext nearest codeword, then the metric m(k) is a small value and thereis a high degree of confidence that the decoded data block is correct.Conversely, if the received codeword has approximately equal distance tothe nearest codeword and the next nearest codeword, then the metric m(k)approaches one, or m(k)→1, and there is less confidence that the decodeddata block is correct.

Equation (2) shows one exemplary metric that is based on the ratio ofEuclidean distances and which may be used to determine whether the blockdecoding of a given received codeword is correct or in error. Othermetrics may also be used for erasure detection, and this is within thescope of the invention. In general, a metric may be defined based on anysuitable reliability function ƒ(r,C), where r is a received codeword andC is a codebook or collection of all possible codewords. The functionƒ(r, C) should be indicative of the quality/reliability of a receivedcodeword and should have the proper characteristic (e.g., monotonic withdetection reliability).

Erasure detection may be performed to determine whether the decodingresult for each received codeword meets a predetermined level ofconfidence. The metric m(k) for a received codeword may be comparedagainst an erasure threshold, TH_(erasure), to obtain a decodingdecision for the received codeword, as follows:m(k)<TH_(erasure), declare a non-erased codeword,m(k)≧TH_(erasure), declare an erased codeword.   Eq (3)

As shown in equation (3), the received codeword is declared as (1) an“erased” codeword if the metric m(k) is equal to or greater than theerasure threshold and (2) a “non-erased” codeword if the metric m(k) isless than the erasure threshold. The base station may treat decoded datablocks for non-erased and erased codewords differently. For example, thebase station may use decoded data blocks for non-erased codewords forsubsequent processing and may discard decoded data blocks for erasedcodewords.

The probability of declaring a received codeword as an erased codewordis called an erasure rate and is denoted as Pr_(erasure). The erasurerate is dependent on the erasure threshold used for erasure detectionand the received signal quality (SNR) for the received codeword. Thesignal quality may be quantified by a signal-to-noise ratio, asignal-to-noise-and-interference ratio, and so on. For a given receivedSNR, a low erasure threshold increases the likelihood of a receivedcodeword being declared an erased codeword, and vice versa. For a givenerasure threshold, a low received SNR also increases the likelihood of areceived codeword being declared an erased codeword, and vice versa. Fora given erasure threshold, the received SNR may be set (by controllingthe transmit power for the control channel, as described below) toachieve the desired erasure rate.

The erasure threshold may be set to achieve the desired performance forthe control channel. For example, a probability of error conditioned onnon-erased codewords, which is called a conditional error rate, may beused for the control channel. This conditional error rate is denoted asPr_(error) and means the following: given that a received codeword isdeclared to be a non-erased codeword, the probability of the decodeddata block for the received codeword being incorrect is Pr_(error). Alow Pr_(error) (e.g., 1% or 0.1%) corresponds to a high degree ofconfidence in the decoding result when a non-erased codeword isdeclared. A low Pr_(error) may be desirable for many types oftransmission where reliable decoding is important. The erasure thresholdmay be set to the proper level to achieve the desired Pr_(error).

A well-defined relationship may be expected to exist between the erasurerate Pr_(erasure), the conditional error rate Pr_(error), the erasurethreshold TH_(erasure), and the received SNR. In particular, for a givenerasure threshold and a given received SNR, there exists a specificerasure rate and a specific conditional error rate. By changing theerasure threshold, a trade off may be made between the erasure rate andthe conditional error rate. Computer simulation may be performed and/orempirical measurements may be made to determine or predict therelationship between the erasure rate and the conditional error rate fordifferent erasure threshold values and different received SNRs.

However, in a practical system, the relationship between these fourparameters may not be known in advance and may be dependent ondeployment scenarios. For example, the specific erasure threshold thatcan achieve the desired erasure rate and conditional error rate may notbe known a priori and may even change over time, but probably slowly.Furthermore, it is not known whether “predicted” relationship betweenthe erasure rate and the conditional error rate, obtained via simulationor by some other means, will hold true in an actual deployment.

A power control mechanism may be used to dynamically adjust the erasurethreshold and the received SNR to achieve the desired performance forthe control channel. The control channel performance may be quantifiedby a target erasure rate Pr_(erasure) (e.g., 10% erasure rate, orPr_(erasure)=0.1) and a target conditional error rate Pr_(error) (e.g.,1% conditional error rate, or Pr_(error)=0.01), i e, a (Pr_(erasure),Pr_(error)) pair.

FIG. 2 shows a power control mechanism 200 that may be used todynamically adjust the erasure threshold and to control the transmitpower for a transmission sent on the control channel from a terminal toa base station. Power control mechanism 200 includes an inner loop 210,an outer loop 220, and a third loop 230.

Inner loop 210 attempts to maintain the received SNR for thetransmission, as measured at the base station, as close as possible to atarget SNR. For inner loop 210, an SNR estimator 242 at the base stationestimates the received SNR for the transmission and provides thereceived SNR to a transmit power control (TPC) generator 244. TPCgenerator 244 also receives the target SNR for the control channel,compares the received SNR against the target SNR, and generates TPCcommands based on the comparison results. Each TPC command is either (1)an UP command to direct an increase in transmit power for the controlchannel or (2) a DOWN command to direct a decrease in transmit power.The base station transmits the TPC commands on the forward link (cloud260) to the terminal.

The terminal receives and processes the forward link transmission fromthe base station and provides “received” TPC commands to a TPC processor262. Each received TPC command is a noisy version of a TPC command sentby the base station. TPC processor 262 detects each received TPC commandand obtains a TPC decision, which may be (1) an UP decision if thereceived TPC command is deemed to be an UP command or (2) a DOWNdecision if the received TPC command is deemed to be a DOWN command.

A transmit (TX) power adjustment unit 264 adjusts the transmit power forthe transmission on the control channel based on the TPC decisions fromTPC processor 262. Unit 264 may adjust the transmit power as follows:$\begin{matrix}{{P_{cch}( {n + 1} )} = \{ \begin{matrix}{{P_{cch}(n)} + {\Delta\quad P_{up}}} & {{{for}\quad{an}\quad{UP}\quad{decision}},} \\{{P_{cch}(n)} - {\Delta\quad P_{dn}}} & {{{for}\quad a{\quad\quad}{DOWN}\quad{decision}},}\end{matrix} } & {{Eq}\quad(4)}\end{matrix}$

-   -   where P_(cch)(n) is the transmit power for inner loop update        interval n;    -   ΔP_(up) is an up step size for the transmit power; and    -   ΔP_(dn) is a down step size for the transmit power.

The transmit power P_(cch)(n) and step sizes ΔP_(up) and ΔP_(dn) are inunits of decibels (dB). As shown in equation (4), the transmit power isincreased by ΔP_(up) for each UP decision and decreased by ΔP_(dn) foreach DOWN decision. Although not described above for simplicity, a TPCdecision may also be a “no-OP” decision if a received TPC command isdeemed to be too unreliable, in which case the transmit power may bemaintained at the same level, or P_(cch)(n +1)=P_(cch)(n). The ΔP_(up)and ΔP_(dn) step sizes are typically equal, and may both be set to 1.0dB, 0.5 dB, or some other value.

Due to path loss, fading, and multipath effects on the reverse link(cloud 240), which typically vary over time and especially for a mobileterminal, the received SNR for the transmission on the control channelcontinually fluctuates. Inner loop 210 attempts to maintain the receivedSNR at or near the target SNR in the presence of changes in the reverselink channel condition.

Outer loop 220 continually adjusts the target SNR such that the targeterasure rate is achieved for the control channel. A metric computationunit 252 computes the metric m(k) for each received codeword obtainedfrom the control channel, as described above. An erasure detector 254performs erasure detection for each received codeword based on thecomputed metric m(k) for the codeword and the erasure threshold andprovides the status of the received codeword (either erased ornon-erased) to a target SNR adjustment unit 256.

Target SNR adjustment unit 256 obtains the status of each receivedcodeword and adjusts the target SNR for the control channel, as follows:$\begin{matrix}{{{SNR}_{target}( {k + 1} )} = \{ \begin{matrix}{{{{SNR}_{target}(k)} + {\Delta\quad{SNR}_{up}}},} & {{{for}\quad{an}\quad{erased}\quad{codeword}},} \\{{{{SNR}_{target}(k)} - {\Delta\quad{SNR}_{dn}}},} & {{{for}\quad a\quad{non}\text{-}{erased}\quad{codeword}},}\end{matrix} } & {{Eq}\quad(5)}\end{matrix}$where

-   -   SNR_(target)(k) is the target SNR for outer loop update interval        k;    -   ΔSNR_(up) is an up step size for the target SNR; and    -   ΔSNR_(dn) is a down step size for the target SNR.

The target SNR SNR_(target)(k) and the step sizes ΔSNR_(up) andΔSNR_(dn) are in units of dB. As shown in equation (5), unit 256 reducesthe target SNR by ΔSNR_(dn) if a received codeword is deemed to be anon-erased codeword, which may indicate that the received SNR for thecontrol channel is higher than necessary. Conversely, unit 256 increasesthe target SNR by ΔSNR_(up) if a received codeword is deemed to be anerased codeword, which may indicate that the received SNR for thecontrol channel is lower than necessary.

The ΔSNR_(up) and ΔSNR_(dn) step sizes for adjusting the target SNR maybe set based on the following relationship: $\begin{matrix}{{\Delta\quad{SNR}_{up}} = {\Delta\quad{{SNR}_{dn} \cdot {( \frac{1 - \Pr_{erasure}}{\Pr_{erasure}} ).}}}} & {{Eq}\quad(6)}\end{matrix}$

For example, if the target erasure rate for the control channel is 10%(or Pr_(erasure)=0.1), then the up step size is 9 times the down stepsize (or ΔSNR_(up)=9·ΔSNR_(dn)). If the up step size is selected to be0.5 decibel (dB), then the down step size is approximately 0.056 dB.Larger values for ΔSNR_(up) and ΔSNR_(dn) speed up the convergence ratefor outer loop 220. A large value for ΔSNR_(up) also causes morefluctuation or variation of the target SNR at steady state.

Third loop 230 dynamically adjusts the erasure threshold such that thetarget conditional error rate is achieved for the control channel. Theterminal may transmit a known codeword on the control channelperiodically or whenever triggered. The base station receives thetransmitted known codeword. Metric computation unit 252 and erasuredetector 254 perform erasure detection for each received known codewordbased on the erasure threshold and in the same manner as for thereceived codewords. For each received known codeword deemed to benon-erased, a decoder 262 decodes the received known codeword anddetermines whether the decoded data block is correct or in error, whichcan be done since the codeword is known. Decoder 262 provides to anerasure threshold adjustment unit 264 the status of each received knowncodeword, which may be: (1) an erased codeword, (2) a “good” codeword ifthe received known codeword is a non-erased codeword and decodedcorrectly, or (3) a “bad” codeword if the received known codeword is anon-erased codeword but decoded in error.

Erasure threshold adjustment unit 264 obtains the status of the receivedknown codewords and adjusts the erasure threshold, as follows:$\begin{matrix}{{{TH}_{erasure}( {\ell + 1} )} = \{ \begin{matrix}{{{{TH}_{erasure}(\ell)} + {\Delta\quad{TH}_{up}}},} & {{{for}\quad a\quad{good}\quad{codeword}},} \\{{{{TH}_{erasure}(\ell)} - {\Delta\quad{TH}_{dn}}},} & {{{for}\quad a\quad{bad}{\quad\quad}{codeword}},{and}} \\{{{TH}_{erasure}(\ell)},} & {{{for}\quad{an}\quad{erased}\quad{codeword}},}\end{matrix} } & {{Eq}\quad(7)}\end{matrix}$

where TH_(erasure)(l) is the erasure threshold for third loop updateinterval l;

-   -   ΔTH_(up) is an up step size for the erasure threshold; and    -   ΔTH_(dn) is a down step size for the erasure threshold.

As shown in equation (7), the erasure threshold is decreased by ΔTH_(dn)for each received known codeword that is a bad codeword. The lowererasure threshold corresponds to a more stringent erasure detectioncriterion and results in a received codeword being more likely to bedeemed erased, which in turn results in the received codeword being morelikely to be decoded correctly when deemed to be non-erased. Conversely,the erasure threshold is increased by ΔTH_(up) for each received knowncodeword that is a good codeword. The higher erasure thresholdcorresponds to a less stringent erasure detection criterion and resultsin a received codeword being less likely to be deemed erased, which inturn results in the received codeword being more likely to be decoded inerror when deemed to be non-erased. The erasure threshold is maintainedat the same level for received known codewords that are erased.

The ΔTH_(up) and ΔTH_(dn) step sizes for adjusting the erasure thresholdmay be set based on the following relationship: $\begin{matrix}{{\Delta\quad{TH}_{dn}} = {\Delta\quad{{TH}_{up} \cdot {( \frac{1 - \Pr_{error}}{\Pr_{error}} ).}}}} & {{Eq}\quad(8)}\end{matrix}$

For example, if the target conditional error rate for the controlchannel is 1%, then the down step size is 99 times the up step size. Themagnitude of ΔTH_(up) and ΔTH_(dn) may be determined based on theexpected magnitude of the received symbols, the desired convergence ratefor the third loop, and possibly other factors.

In general, the adjustment of the erasure threshold is dependent on howthe metric used for erasure detection is defined. Equations (7) and (8)are based on the metric defined as shown in equation (2). The metric mayalso be defined in other manners (e.g., m(k)=d_(n2)(k)/d_(n1)(k) insteadof m(k)=d_(n1)(k)/d_(n2)(k)), in which case the adjustment of theerasure threshold may be modified accordingly. The adjustable erasurethreshold may also be used in combination with any erasure detectiontechnique to achieve robust erasure detection performance for variouschannel conditions.

The erasure threshold, TH_(erasure)(l), may be dynamically adjusted invarious manners. In one embodiment, a separate third loop is maintainedby the base station for each terminal in communication with the basestation. This embodiment allows the erasure threshold to be individuallyadjusted for each terminal, which then allows the control channelperformance to be specifically tailored for the terminal. For example,different terminals may have different target conditional error rates,which may be achieved by operating separate third loops for theseterminals. In another embodiment, a single third loop is maintained bythe base station for all terminals in communication with the basestation. The common erasure threshold is then used for erasure detectionfor all of these terminals and is also updated based on known codewordsreceived by the base station from these terminals. This embodimentprovides good performance for all terminals if the control channelperformance is robust for these terminals for various channelconditions. This embodiment allows for a faster rate of convergence forthe third loop and also reduces overhead since each terminal maytransmit the known codeword at a lower rate (e.g., once every fewhundred milli-seconds). In yet another embodiment, a single third loopis maintained by the base station for each group of terminals having thesame control channel performance, and the erasure threshold is updatedbased on known codewords received by the base station from all terminalsin the group.

Inner loop 210, outer loop 220, and third loop 230 are typically updatedat different rates. Inner loop 210 is the fastest loop of the threeloops, and the transmit power for the control channel may be updated ata particular rate (e.g., 150 times per second). Outer loop 220 is thenext fastest loop, and the target SNR may be updated whenever a codewordis received on the control channel. Third loop 230 is the slowest loop,and the erasure threshold may be updated whenever a known codeword isreceived on the control channel. The update rates for the three loopsmay be selected to achieve the desired performance for erasure detectionand power control.

For the embodiment described above, the target conditional error ratePr_(error) is used as one of the measures of performance for the controlchannel, and the third loop is designed to achieve this Pr_(error).Other measures of performance may also be used for the control channel,and the third loop may be designed accordingly. For example, a targetprobability of a received codeword being decoded in error when deemed tobe erased may be used for the third loop.

FIGS. 3A and 3B show a flow diagram of a process 300 for updating thesecond and third loops of power control mechanism 300. A receivedcodeword k is initially obtained from the control channel (block 312).The metric m(k) is computed for the received codeword, e.g., asdescribed above, (block 314) and compared against the erasure threshold(block 316). If the computed metric m(k) is greater than or equal to theerasure threshold, as determined in block 320, and if the receivedcodeword is not a known codeword, as determined in block 322, then thereceived codeword is declared as an erased codeword (block 324). Thetarget SNR is increased by the ΔSNR_(up) step size if the computedmetric m(k) is greater than or equal to the erasure threshold,regardless of whether the received codeword is known or not known (block326). After block 326, the process returns to block 312 to process thenext received codeword.

If the computed metric m(k) is less than the erasure threshold, asdetermined in block 320, and if the received codeword is not a knowncodeword, as determined in block 332, then the received codeword isdeclared as a non-erased codeword (block 334), and the target SNR isdecreased by the ΔSNR_(dn) step size (block 336). The process returns toblock 312 to process the next received codeword.

If the computed metric m(k) is less than the erasure threshold, asdetermined in block 320, and if the received codeword is a knowncodeword, as determined in block 332, then (referring to FIG. 3B) thereceived codeword is decoded (block 340). If the decoding was correct,as determined in block 342, then the received known codeword is declaredas a good codeword (block 344), and the erasure threshold is increasedby the ΔTH_(up) step size (block 346). Otherwise, if there was adecoding error, as determined in block 342, then the received knowncodeword is declared as a bad codeword (block 354), and the erasurethreshold is decreased by the ΔTH_(dn) step size (block 356). Fromblocks 346 and 356, the process returns to block 312 in FIG. 3A toprocess the next received codeword.

As noted above, the techniques described herein may be used for varioustypes of physical channels that do not employ error detection coding.The use of these techniques for an exemplary data transmission scheme isdescribed below. For this transmission scheme, a terminal desiring aforward link transmission estimates the received signal quality of theforward link for its serving base station (e.g., based on a pilottransmitted by the base station). The received signal quality estimatemay be translated to an L-bit value, which is called a channel qualityindicator (CQI). The CQI may indicate the received SNR for the forwardlink, the supported data rate for the forward link, and so on. In anycase, block coding is performed on the CQI to obtain a CQI codeword. Asa specific example, L may be equal to 4, and the CQI codeword maycontain 16 QPSK modulation symbols, or [s_(i)(1) s_(i)(2) . . .s_(i)(16)]. The terminal transmits the CQI codeword on the CQI channel(which is one of the control channels) to the serving base station. Theserving base station receives the CQI codeword sent on the CQI channeland performs erasure detection on the received CQI codeword. If thereceived CQI codeword is not erased, then the serving base stationdecodes the received CQI codeword and uses the decoded CQI to schedule adata transmission for the terminal.

FIG. 4 shows a set of data and control channels used for the exemplarydata transmission scheme. The terminal measures the received signalquality of the forward link and transmits a CQI codeword on the CQIchannel. The terminal continually makes measurements of the forward linkquality and sends updated CQI codewords on the CQI channel. Thus,discarding received CQI codewords deemed to be erased is not detrimentalto system performance. However, received CQI codewords deemed to benon-erased should be of high quality since a forward link transmissionmay be scheduled based on the information contained in these non-erasedCQI codewords.

If the terminal is scheduled for forward link transmission, then theserving base station processes data packets to obtain coded packets andtransmits the coded packets on a forward link data channel to theterminal. For a hybrid automatic retransmission (H-ARQ) scheme, eachcoded packet is partitioned into multiple subblocks, and one subblock istransmitted at a time for the coded packet. As each subblock for a givencoded packet is received on the forward link data channel, the terminalattempts to decode and recover the packet based on all subblocksreceived thus far for the packet. The terminal is able to recover thepacket based on a partial transmission because the subblocks containredundant information that is useful for decoding when the receivedsignal quality is poor but may not be needed when the received signalquality is good. The terminal then transmits an acknowledgment (ACK) onan ACK channel if the packet is decoded correctly, or a negativeacknowledgment (NAK) otherwise. The forward link transmission continuesin this manner until all coded packets are transmitted to the terminal.

The techniques described herein may be advantageously used for the CQIchannel. Erasure detection may be performed on each received CQIcodeword as described above. The transmit power for the CQI channel maybe adjusted using power control mechanism 300 to achieve the desiredperformance for the CQI channel (e.g., the desired erasure rate and thedesired conditional error rate). The transmit power for other controlchannels (e.g., the ACK channel) and reverse link data channels may alsobe set based on the power-controlled transmit power for the CQI channel.

For clarity, the erasure detection and power control techniques havebeen specifically described for the reverse link. These techniques mayalso be used for erasure detection and power control for a transmissionsent on the forward link.

FIG. 5 shows a block diagram of an embodiment of a base station 110 xand a terminal 120 x. On the reverse link, at terminal 120 x, a transmit(TX) data processor 510 receives and processes (e.g., formats, codes,interleaves, and modulates) reverse link (RL) traffic data and providesmodulation symbols for the traffic data. TX data processor 510 alsoprocesses control data (e.g., CQI) from a controller 520 and providesmodulation symbols for the control data. A modulator (MOD) 512 processesthe modulation symbols for traffic and control data and pilot symbolsand provides a sequence of complex-valued chips. The processing by TXdata processor 510 and modulator 512 is dependent on the system. Forexample, modulator 512 may perform OFDM modulation if the systemutilizes OFDM. A transmitter unit (TMTR) 514 conditions (e.g., convertsto analog, amplifies, filters, and frequency upconverts) the sequence ofchips and generates a reverse link signal, which is routed through aduplexer (D) 516 and transmitted via an antenna 518.

At base station 110 x, the reverse link signal from terminal 120 x isreceived by an antenna 552, routed through a duplexer 554, and providedto a receiver unit (RCVR) 556. Receiver unit 556 conditions (e.g.,filters, amplifies, and frequency downconverts) the received signal andfurther digitizes the conditioned signal to obtain a stream of datasamples. A demodulator (DEMOD) 558 processes the data samples to obtainsymbol estimates. A receive (RX) data processor 560 then processes(e.g., deinterleaves and decodes) the symbol estimates to obtain decodeddata for terminal 120 x. RX data processor 560 also performs erasuredetection and provides to a controller 570 the status of each receivedcodeword used for power control. The processing by demodulator 558 andRX data processor 560 is complementary to the processing performed bymodulator 512 and TX data processor 510, respectively.

The processing for a forward link transmission may be performedsimilarly to that described above for the reverse link. The processingfor reverse link and forward link transmissions is typically specifiedby the system.

For reverse link power control, an SNR estimator 574 estimates thereceived SNR for terminal 120 x and provides the received SNR to a TPCgenerator 576. TPC generator 576 also receives the target SNR andgenerates TPC commands for terminal 120 x. The TPC commands areprocessed by a TX data processor 582, further processed by a modulator584, conditioned by a transmitter unit 586, routed through duplexer 554,and transmitted via antenna 552 to terminal 120 x.

At terminal 120 x, the forward link signal from base station 110 x isreceived by antenna 518, routed through duplexer 516, conditioned anddigitized by a receiver unit 540, processed by a demodulator 542, andfurther processed by an RX data processor 544 to obtain received TPCcommands. A TPC processor 524 then detects the received TPC commands toobtain TPC decisions, which are used to generate a transmit poweradjustment control. Modulator 512 receives the control from TPCprocessor 524 and adjusts the transmit power for the reverse linktransmission. Forward link power control may be achieved in a similarmanner.

Controllers 520 and 570 direct the operations of various processingunits within terminal 120 x and base station 110 x, respectively.Controller 520 and 570 may also perform various functions for erasuredetection and power control for the forward link and reverse link. Forexample, each controller may implement the SNR estimator, TPC generator,and target SNR adjustment unit for its link. Controller 570 and RX dataprocessor 560 may also implement process 300 in FIGS. 3A and 3B. Memoryunits 522 and 572 store data and program codes for controllers 520 and570, respectively.

The erasure detection and power control techniques described herein maybe implemented by various means. For example, these techniques may beimplemented in hardware, software, or a combination thereof. For ahardware implementation, the processing units used to perform erasuredetection and/or power control may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof.

For a software implementation, the techniques described herein may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory unit (e.g., memory unit 572 in FIG. 5) and executed by aprocessor (e.g., controller 570). The memory unit may be implementedwithin the processor or external to the processor, in which case it canbe communicatively coupled to the processor via various means as isknown in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An apparatus operable to perform erasure detection in a wirelesscommunication system, comprising: a metric computation unit operative toobtain received codewords for codewords transmitted via a wirelesschannel and to compute a metric for each of the received codewords,wherein each transmitted codeword is a block of coded or uncoded data,and wherein each received codeword is a noisy version of a transmittedcodeword; an erasure detector operative to compare the computed metricfor each received codeword against an erasure threshold and to declareeach received codeword to be an erased codeword or a non-erased codewordbased on a comparison result for the received codeword; and anadjustment unit operative to dynamically adjust the erasure threshold toachieve a target level of performance for erasure detection.
 2. Theapparatus of claim 1, further comprising: a decoder operative to obtainreceived known codewords for known codewords transmitted via thewireless channel, each known codeword being a block of a known data, andeach received known codeword being a noisy version of a transmittedknown codeword, decode each received known codeword deemed to be anon-erased codeword, and determine a status of each received knowncodeword as being a good codeword, a bad codeword, or an erasedcodeword, a good codeword being a received known codeword declared to bea non-erased codeword and decoded correctly, and a bad codeword being areceived known codeword declared to be a non-erased codeword but decodedin error, and wherein the adjustment unit is operative to adjust theerasure threshold based on the status of each received known codeword.3. An apparatus operable to perform erasure detection in a wirelesscommunication system, comprising: means for obtaining received codewordsfor codewords transmitted via a wireless channel, each transmittedcodeword being a block of coded or uncoded data, and each receivedcodeword being a noisy version of a transmitted codeword; means forcomputing a metric for each of the received codewords; means forcomparing the computed metric for each received codeword against anerasure threshold; means for declaring each received codeword to be anerased codeword or a non-erased codeword based on a comparison resultfor the received codeword; and means for dynamically adjusting theerasure threshold to achieve a target level of performance for erasuredetection.
 4. The apparatus of claim 3, further comprising: means forobtaining received known codewords for known codewords transmitted viathe wireless channel, each known codeword being a block of a known data,and each received known codeword being a noisy version of a transmittedknown codeword; means for determining a status of each of the receivedknown codewords as being a good codeword, a bad codeword, or an erasedcodeword, a good codeword being a received known codeword declared to bea non-erased codeword and decoded correctly, and a bad codeword being areceived known codeword declared to be a non-erased codeword but decodedin error; and means for adjusting the erasure threshold based on thestatus of each received known codeword.
 5. An apparatus operable toperform power control for a transmission sent via a wireless channel ina wireless communication system, comprising: a data processor operativeto obtain received codewords for codewords transmitted in thetransmission, each transmitted codeword being a block of coded oruncoded data, and each received codeword being a noisy version of atransmitted codeword, determine a status of each received codeword asbeing an erased codeword or a non-erased codeword based on a metriccomputed for the received codeword and an erasure threshold, obtainreceived known codewords for known codewords transmitted via thewireless channel, each known codeword being a block of a known data, andeach received known codeword being a noisy version of a transmittedknown codeword, and determine a status of each received known codewordas being a good codeword, a bad codeword, or an erased codeword, a goodcodeword being a received known codeword deemed to be a non-erasedcodeword and decoded correctly, and a bad codeword being a receivedknown codeword deemed to be a non-erased codeword but decoded in error;and a controller operative to adjust a target signal quality (SNR) basedon the status of each received codeword, wherein transmit power for thetransmission is adjusted based on the target SNR, and adjust the erasurethreshold based on the status of each received known codeword.
 6. Theapparatus of claim 5, further comprising: an SNR estimator operative toestimate a received SNR for the transmission; and a generator operativeto compare the received SNR against the target SNR and generate commandsused to adjust the transmit power for the transmission.
 7. The apparatusof claim 5, wherein the controller is operative to adjust the erasurethreshold to achieve a target conditional error rate indicative of apredetermined probability of a received codeword being decoded in errorif declared to be a non-erased codeword.
 8. The apparatus of claim 5,wherein the controller is operative to adjust the target SNR to achievea target erasure rate indicative of a predetermined probability ofdeclaring a received codeword as an erased codeword.
 9. The apparatus ofclaim 5, wherein the transmission is for a control channel.
 10. Theapparatus of claim 9, wherein the control channel is used to sendchannel quality information, and wherein each transmitted codeword isfor a channel quality indicator.
 11. The apparatus of claim 5, whereinthe received known codewords are obtained from a plurality of differenttransmitting entities.
 12. The apparatus of claim 5 and utilized in abase station.
 13. The apparatus of claim 5 and utilized in a wirelessterminal.
 14. An apparatus operable to perform power control for atransmission sent via a wireless channel in a wireless communicationsystem, comprising: means for obtaining received codewords for codewordstransmitted in the transmission, each transmitted codeword being a blockof coded or uncoded data, and each received codeword being a noisyversion of a transmitted codeword; means for determining a status ofeach received codeword as being an erased codeword or a non-erasedcodeword based on a metric computed for the received codeword and anerasure threshold; means for adjusting a target signal quality (SNR)based on the status of each received codeword, wherein transmit powerfor the transmission is adjusted based on the target SNR; means forobtaining received known codewords for known codewords transmitted viathe wireless channel, each known codeword being a block of a known data,and each received known codeword being a noisy version of a transmittedknown codeword; means for determining a status of each received knowncodeword as being a good codeword, a bad codeword, or an erasedcodeword, a good codeword being a received known codeword deemed to be anon-erased codeword and decoded correctly, and a bad codeword being areceived known codeword deemed to be a non-erased codeword but decodedin error; and means for adjusting the erasure threshold based on thestatus of each received known codeword.
 15. The apparatus of claim 14,further comprising: means for estimating a received SNR for thetransmission; means for comparing the received SNR against the targetSNR; and means for generating commands based on results of thecomparing, wherein the commands are used to adjust the transmit powerfor the transmission.